We report a kinematically complete experiment on the production of ${\mathrm{CO}}^{+}$ autoionizing states following photoionization of carbon monoxide below its vertical double-ionization threshold. Momentum imaging spectroscopy is used to measure the energies and body-frame angular distributions of both photo- and autoionization electrons, as well as the kinetic energy release (KER) of the atomic ions. These data, in combination with ab initio theoretical calculations, provide insight into the nature of the cation states produced and their subsequent dissociation into autoionizing atomic (${\mathrm{O}}^{*}$) fragments.

Compression of UV femtosecond laser pulses focused into a gas cell filled with Xe is reported numerically. With a large negative Kerr index and normal dispersion, Xe promotes temporal modulational instability (MI), which can be monitored to shorten $~100$ fs pulses to robust, singly peaked waveforms exhibiting a fourfold compression factor. Combining standard MI theory with a variational approach allows us to predict the beam parameters suitable for efficient compression. At powers $⩽30$ MW, nonlinear dispersion is shown to shift the pulse temporal profile to the rear zone.

We show, by means of simple model calculations, how a weak laser beam sent though an optical fiber exerts a transverse radiation force if there is an azimuthal asymmetry present in the fiber such that one side has a slightly different refractive index than the other. The refractive index difference $\Delta n$ needs only to be very low, of order ${10}^{-3}$, to produce an appreciable transverse displacement of order 10 $\mu$m. We argue that the effect has probably already been seen in a recent experiment by W. She et al. [Phys. Rev. Lett. 101, 243601 (2008)], and we discuss the correspondence between these observations and the theory presented. The effect could be used to bend optical fibers in a predictable and controlled manner and we propose that it could be useful for micron-scale devices.

We propose a complete quantum computation process on a topologically protected Josephson junction array system, originally proposed by Ioffe and Feigel’man [Phys. Rev. B 66, 224503 (2002)]. Logical qubits for computation are encoded in the punctured array. The number of qubits is determined by the number of holes. The topological degeneracy is lightly shifted by tuning the flux along specific paths. We show how to perform both single-qubit and basic quantum-gate operations in this system, especially the controlled-NOT (CNOT) gate.

This article begins with a simple proof of the existence of squash operators compatible with the Bennett-Brassard 1984 (BB84) protocol that suits single-mode as well as multimode threshold detectors. The proof shows that, when a given detector is symmetric under cyclic group $C_4$, and a certain observable associated with it has rank two as a matrix, then there always exists a corresponding squash operator. Next, we go on to investigate whether the above restriction of “rank two” can be eliminated; i.e., is cyclic symmetry alone sufficient to guarantee the existence of a squash operator? The motivation behind this question is that, if this were true, it would imply that one could realize a device-independent and unconditionally secure quantum key distribution protocol. However, the answer turns out to be negative, and moreover, one can instead prove a no-go theorem that any symmetry is, by itself, insufficient to guarantee the existence of a squash operator.

The adiabatic connection of density functional theory (DFT) for electronic systems is generalized here to negative values of the coupling strength $\alpha$ (with attractive electrons). In the extreme limit $\alpha \to -\infty$ a simple physical solution is presented and its implications for DFT (as well as its limitations) are discussed. For two-electron systems (a case in which the present solution can be calculated exactly), we find that an interpolation between the limit $\alpha \to -\infty$ and the opposite limit of infinitely strong repulsion ($\alpha \to +\infty$) yields a rather accurate estimate of the second-order correlation energy $E_{\mathrm{c}}^{\mathrm{GL}2}[\rho ]$ for several different densities $\rho$, without using virtual orbitals. The same procedure is also applied to the Be isoelectronic series, analyzing the effects of near degeneracy.

In this work, we report a theoretical study on $e^{-}$–CS${}_2$ collisions in the low- and intermediate-energy ranges. Elastic differential, integral, and momentum-transfer cross sections, as well as grand total (elastic $+$ inelastic) and absorption cross sections, are reported in the $1-1000$ eV range. A recently proposed complex optical potential composed of static, exchange, and correlation-polarization plus absorption contributions is used to describe the electron–molecule interaction. The Schwinger variational iterative method combined with the distorted-wave approximation is applied to calculate the scattering amplitudes. The comparison between our calculated results and the existing experimental and/or theoretical results is encouraging.

The scattering of relativistic spin-1/2 neutral particles with a magnetic dipole moment by a long straight charged line and a magnetic flux line at the same position is studied. The scattering cross sections for unpolarized and polarized particles are obtained by solving the Dirac-Pauli equation. The results are in general the same as those for pure Aharonov-Casher scattering (by the charged line alone) as expected. However, in special cases when the incident energy, the line charge density, and the magnetic flux satisfy some relations, the cross section for polarized particles is dramatically changed. Relations between the polarization of incident particles and that of scattered ones are presented, both in the full relativistic case and the nonrelativistic limit. The characteristic difference between the general and special cases lies in the backward direction: in the general cases the incident particles are simply bounced while in the special cases their polarization is turned over simultaneously. For pure Aharonov-Casher scattering there exist cases where the helicities of all scattered particles are reversed. This seems to be remarkable but appears unnoticed previously. Two mathematical approaches are employed to deal with the singularity of the electric and magnetic field and it turns out that the physical results are essentially the same.

Cold atoms in optical lattices have emerged as an ideal system to investigate the ratchet effect, as demonstrated by several recent experiments. In this work we analyze theoretically two aspects of ac-driven transport in cold-atom ratchets. We first address the issue of whether, and to what extent, an ac-driven ratchet for cold atoms can operate as a motor. We thus study theoretically a dissipative motor for cold atoms, as obtained by adding a load to a 1D nonadiabatically driven rocking ratchet. We demonstrate that a current can be generated also in the presence of a load, e.g., the ratchet device can operate as a motor. Correspondingly, we determine the stall force for the motor, which characterizes the range of loads over which the device can operate as a motor, and the differential mobility, which characterizes the response to a change in the magnitude of the load. Second, we compare our results for the transport in an ac-driven ratchet device with the transport in a dc-driven system. We observe a peculiar phenomenon: the double-harmonic ac force stabilizes the dynamics, allowing the generation of uniform directed motion over a range of momentum much larger than what is possible with a dc bias. We explain such a stabilization of the dynamics by observing that a nonadiabatic ac drive broadens the effective cooling momentum range and forces the atom trajectories to cover such a region. Thus the system can dissipate energy and maintain a steady-state energy balance. Our results show that in the case of a finite-range velocity-dependent friction, a ratchet device may offer the possibility of controlling the particle motion over a broader range of momentum with respect to a purely biased system, although this is at the cost of a reduced coherency.

In this article, we investigate the dependence of interatomic Coulombic decay widths on the symmetry of the decaying state. In this type of decay, excited, ionized, and doubly ionized states of an atom or molecule can efficiently relax by ionizing their environment. We concentrate on an atom $A$ and a neighboring atom $B$ and consider such excited, ionized, or doubly ionized states of $A$ that decay by emitting a single photon if $A$ were an isolated atom. Analytical expressions for the various widths are derived for large interatomic distances $R$. A pronounced dependence of the widths on the symmetry properties of the decaying state is found. This dependence at large $R$ is related to the dependence of the interaction energy of two classical dipoles on their mutual orientation. Comparison with precise ab initio calculations shows that the analytical results hold well at large $R$, while they deviate from the ab initio values at smaller $R$ due to the effect of orbital overlap.

In this work, we investigate the theory for three different unidirectional population-transfer schemes in trapped multilevel systems which can be utilized to cool molecular ions. The approach we use exploits the laser-induced coupling between the internal and motional degrees of freedom so that the internal state of a molecule can be mapped onto the motion of that molecule in an external trapping potential. By sympathetically cooling the translational motion back into its ground state, the mapping process can be employed as part of a cooling scheme for molecular rotational levels. This step is achieved through a common mode involving a laser-cooled atom trapped alongside the molecule. For the coherent mapping, we will focus on adiabatic passage techniques which may be expected to provide robust and efficient population transfers. By applying far-detuned chirped adiabatic rapid passage pulses, we are able to achieve an efficiency of better than $98\%$ for realistic parameters and including spontaneous emission. Even though our main focus is on cooling molecular states, the analysis of the different adiabatic methods has general features which can be applied to atomic systems.

We study the dynamics of a supersonic molecular beam in a low-finesse optical cavity and demonstrate that most molecules in the beam can be decelerated to zero central velocity by the intracavity optical field in a process analogous to electrostatic Stark deceleration. We show that the rapid switching of the optical field for slowing the molecules is automatically generated by the cavity-induced dynamics. We further show that $~$1% of the molecules can be optically trapped at a few millikelvin in the same cavity.

The effect of collectivity on the loading of an atom laser via optical pumping is discussed. In our model, atoms in a beam are laser-excited and subsequently spontaneously decay into a trapping state. We consider the case of sufficiently high particle density in the beam such that the spontaneous emission is modified by the particle interaction. We show that the collective effects lead to a better population of the trapping state over a wide range of system parameters, and that the second-order correlation function of the atoms can be controlled by the applied laser field.

The symmetry-breaking magnetization dynamics of a spin-1 Bose-Einstein condensate (BEC) due to the dipole-dipole interaction are investigated using the mean-field and Bogoliubov theories. When a magnetic field is applied along the symmetry axis of a pancake-shaped BEC in the $m=0$ hyperfine sublevel, transverse magnetization develops breaking the chiral or axial symmetry. A variety of magnetization patterns are formed depending on the strength of the applied magnetic field. The proposed phenomena can be observed in ${}^{87}\mathrm{Rb}$ and ${}^{23}\mathrm{Na}$ condensates.

We study the spatial coherence properties of the entangled two-photon field produced by parametric down-conversion (PDC) when the pump field is, spatially, a partially coherent beam. By explicitly treating the case of a pump beam of the Gaussian Schell-model type, we show that in PDC the spatial coherence properties of the pump field get entirely transferred to the spatial coherence properties of the down-converted two-photon field. As one important consequence of this study, we find that, for two-qubit states based on the position correlations of the two-photon field, the maximum achievable entanglement, as quantified by concurrence, is bounded by the degree of spatial coherence of the pump field. These results could be important by providing a means of controlling the entanglement of down-converted photons by tailoring the degree of coherence of the pump field.

We study the phase-dependent propagation of a strong, resonant pump and two weak symmetrically detuned fields in a two-level system with population decay through a cascade of intermediate levels. As this system forms a closed loop, the propagation is phase-dependent. For an initial total phase $\Phi =0$, there is constructive interference between the two weak fields, leading to parametric amplification on propagation. When $\Phi =\pi$, destructive interference occurs, leading to absorption of the weak fields on propagation. When the weak fields are initially equal in intensity, and $\Phi =0,\pi$, $\Phi$ remains constant on propagation. For other initial phases, $\Phi$ changes on propagation. Dramatic phase changes from $\pi$ to 0 can occur when the weak fields are initially unequal in intensity and $\Phi =\pi$.

Molecular wave functions are used to calculate the harmonic emission of diatomic molecules based on a time-dependent Schrödinger equation. Tunable effects are shown for both magnitudes and molecular-alignment-dependent properties of the harmonic emission with different electronic excited states or mixed states as the initial states. The calculated results are consistent with the experimental results of harmonic signals from ${\mathrm{N}}_2$ under different rotational temperatures and laser intensities. The molecular-alignment dependence of harmonic signals is rapidly weakened with increasing levels of electronic excited states. Especially when the initial state of ${\mathrm{N}}_2$ is the first electronic excited state, $B^3\hspace{0.5em}{\Pi }_g$, the magnitudes of the 21st harmonic are significantly changed, and the molecular-alignment dependence of the harmonic shows reverse properties compared to those of the ground state, $X^1\hspace{0.5em}{\sum }_g^{+}$, which can be used to manipulate the harmonic emission.

We present a procedure for the measurement of quadrature components of an electromagnetic field in the far field as an alternative to the traditional approach based on the homodyne detection (HD) technique. For that we suggest to use coherent sources such as phase-locked lasers or optical parametric oscillators operating above threshold. Then we show how to arrange the detection procedure in the far field that is exactly or partly equivalent to the HD. Our scheme can be applied for both the classical and nonclassical fields. The potential of the procedure is illustrated by an example which utilizes the pixellized sources of the nonclassical light. As an integral part of our investigation we develop a theory of the pixellized source of the spatiotemporally squeezed light.

We consider a finite number of trapped bosons in one dimension (1D) interacting through the harmonic Calogero interaction. Using an exact analytic solution for the energy eigenvalues we calculate the condensate fraction, ground-state energy, and specific heat of the system. For weak interaction the behavior of thermodynamic properties is close to that of noninteracting trapped bosons.

The influence of the environment in the thermal equilibrium properties of a bipartite continuous variable quantum system is studied. The problem is treated within a system-plus-reservoir approach. The considered model reproduces the Brownian motion when the two particles are isolated and induces an effective interaction between them, depending on the choice of the spectral function of the bath. The coupling between the system and the environment guarantees the translational invariance of the system in the absence of an external potential. The entanglement between the particles is measured by the logarithmic negativity, which is shown to monotonically decrease with the increase of the temperature. A range of finite temperatures is found in which entanglement is still induced by the reservoir.